Saturday, August 5, 2017

In the post linked above I've discuss a method to correlate the sensor resistance to a gas ppm using the datasheet curve named "sensitivity characteristics of the MQ-135" and the formula:ppm=a*(Rs/Ro)^b.
Someone ask me about sensor and correlation coefficients of various gasses, so here I will post some correlation coefficient for the a few MQ sensor and gasses.

sensor

gas

a

b

min Rs/Ro

max Rs/Ro

MQ2

LPG

591.283

-2.076502

0.256166

1.68543

MQ3

alcohol

0.3923202

-1.493249

0.1143375

2.497754

MQ4

CH4

1041.333

-2.729007

0.4365277

1.830508

MQ5

CH4

217.4972

-2.422111

0.2058715

1.035233

MQ6

LPG

940.2178

-2.521573

0.3915661

1.847477

MQ7

H2

64.86522

-1.405261

0.05323301

1.203488

MQ8

H2

1079.683

-0.6416874

0.03115283

13.84067

MQ135

CO2

110.3794

-2.721706

0.8038119

2.416431

Those values have been found using the method proposed in the above post, and the figures posted below.
In order to use the correlation function, you now need to compute the Ro value. You can find this value reading the sensor resistance at a know amount of ppm, compy in the gases ppm limits of the datasheet, and then use this formula:Ro=Rs*(a/ppm)^(1/b).